Environmental Engineering Reference
In-Depth Information
has been found to be important in the ocean and more generally in highly anaerobic envir-
onments that have low supplies of organic energy sources.
Hydrologic Losses
As mentioned earlier, the need for primary producers to take up simple, soluble forms
of nutrients greatly complicates nutrient cycling in ecosystems as it creates the possibility
of hydrologic losses. For N, interest in hydrologic losses centers on NO
3
2
, which is highly
mobile due to its negative charge (see earlier). However, NH
4
1
is also highly soluble and
can be lost at high rates, especially in flowing waters, which have less opportunity to inter-
act with positively charged soil particles. There is also interest in loss of dissolved organic
N (DON), which consists of a wide range of compounds produced during the decomposi-
tion of organic matter or synthesized by primary producers. Some components of DON
are highly labile and can degrade to release NH
4
1
and NO
3
2
in receiving waters, while
others are highly humified (products of decomposition, highly resistant to decay) and sta-
ble. Because most DON may not be directly available to plants, it may represent an uncon-
trollable form of hydrologic loss that contributes to N limitation of ecosystem productivity
over the long term.
Given the importance of N for ecosystem productivity, especially in temperate terres-
trial ecosystems, there has been extensive study of the factors regulating hydrologic losses
of N. Many studies have focused on how disturbance of plant uptake by clear-cutting of
forests leads to increases in hydrologic losses and on the plant, soil, and microbial factors
that control the magnitude of these losses. Other research has focused on the ability of ter-
restrial ecosystems to absorb increasing amounts of N deposition from the atmosphere,
which has increased along with fossil fuel combustion, and prevent its movement into
downstream waters sensitive to N-induced eutrophication. And a vast body of research
has focused on minimizing hydrologic losses from intensively fertilized agricultural fields,
including efforts to use wetlands as denitrification “sinks” for excess N moving out of
fields in surface runoff or ground-water flow (
Mitsch et al. 2001
).
NITROGEN CYCLING IN TERRESTRIAL
ECOSYSTEMS
Pools and Fluxes
In nearly all terrestrial ecosystems, the largest pool of N is in the soil (
Table 7.3
). Most
of this N is tied up in recalcitrant soil organic matter (the long-term residual by-product of
decomposition) and plays little role in N cycling between plants and the soluble, available
pool. The available pool is very small and dynamic over the timescale of days, with miner-
alization constantly adding N to this pool and plant uptake and immobilization constantly
removing N. Plant biomass (living and dead) represents the next largest pool of N in
